How would I find, or possibly make, a difficult to find power supply?
May 5, 2020 12:37 PM   Subscribe

I am soon to be the owner of a used synthesizer. Unfortunately it has a European power supply, and I am in the United States. I could of course buy a step-up travel converter, but I’ve found these unreliable and would not want to damage my device. This synth has a “backpack” style transformer and a six pin connection that could easily he re-wired. However, it has some strange requirements...

From a service manual, the PSU specs are:
2 x 15V 0,42 A
9 V 1,6 A

The connector has 5 pins:
1 - +9 V
2 - +9 V
3 - +16 V before the 7812 power regulator, behind it should be +12 V
4 - gnd
5 - -16 V before the 7912 power regulator, behind it should be +12 V

My questions are:
- Would I he able to find a reliable, long-term-use step up converter?
- Can I find an off-the shelf-solution? Like a wall wart with two different outputs?
- If not, how might I go about this?
- Is there a misprint between the specs and pins? Spec says 1x 9v, but pinout seems to be the converse, i.e. 2x 9v
- What is the difference between -16V versus ground?
- Why are there separate 9v connections?

My synth and I thank you in advance for helping get the juice it needs :)
posted by kyliej to Technology (18 answers total) 3 users marked this as a favorite
Could you post images of the service information somewhere? I'd like to see the schematic of the input power regulation. It sounds like you have that?
posted by fritley at 12:44 PM on May 5, 2020

my spouse has had to search for rare synth related equipment and has been part of an online interest community that helped him find it. I don't know it offhand but if you memail me I can send you the name of the community.
posted by fingersandtoes at 12:47 PM on May 5, 2020 [1 favorite]

Response by poster: @fritley I don’t believe the service manual had schematics specifically for the PSU, but there is a copy here:
posted by kyliej at 12:53 PM on May 5, 2020

Best answer: There's nothing wrong with using a good quality step up transformer for a relatively low current draw product like this. That's your best and safest solution in terms of low risk of damaging the synth. Yes, low bid "travel converters" can be junk, but that's not the only option.

It looks like the 16 volt supply is bipolar, so you'd need to find a supply with + and - 16 volt outputs, plus the 9 volt supply. Chances are that the two 9V pins are just doubled up because of the higher current draw. (i.e. the wire from the power supply is thin, so they just use two of them in parallel to reduce voltage drop.) A power supply with these voltages not exactly a common off-the-shelf item. I'm guessing this is a vintage synth and the original PSU is an unregulated transformer-bridge rectifiers-capacitors type supply, so yes, you could build something using one or two transformers + associated parts in a box, but that's a lot of work to go to for little gain. Just my 2 cents.
posted by Larry David Syndrome at 1:01 PM on May 5, 2020 [3 favorites]

Also, to clarify per your question, the -16 volt supply is negative relative to ground and the +16v supply is positive relative to ground. They are essentially two separate mirror-imaged power supplies with a shared ground. If you measure from the +16 to the -16, you'll get 32V.
posted by Larry David Syndrome at 1:07 PM on May 5, 2020 [1 favorite]

Waldorf is still around and I can't imagine they wouldn't be able to answer your question or point you towards an appropriate replacement power supply.
posted by niicholas at 1:08 PM on May 5, 2020 [1 favorite]

Woof. I feel barely qualified to give a response, but I'd be tempted to squeak by on a desktop computer power supply which will get you +-12VDC and then use a 7809 and 7909 for the 9V outputs.
posted by Dmenet at 1:09 PM on May 5, 2020

A PSU of this age might be suspect anyway: electrolytic capacitors have a finite life, and the ones from late 80s - mid 90s were especially crap.
posted by scruss at 1:15 PM on May 5, 2020 [1 favorite]

Woof. I feel barely qualified to give a response, but I'd be tempted to squeak by on a desktop computer power supply which will get you +-12VDC and then use a 7809 and 7909 for the 9V outputs.

Sorry, but this is totally wrong. You need +16VDC and -16VDC (+15 and -15 will likely be sufficient) as well as +9VDC (only, not -9VDC which a 7909 would provide). Computer PSUs will do +12 and -12, but you need more because of the regulators in the synth which you don't want to bypass because of SPSU noise.

I agree with Larry David Syndrome that a small step-up transformer will probably be the easiest option if you can't get hold of an US-spec supply
posted by Stoneshop at 1:44 PM on May 5, 2020 [1 favorite]

Best answer: The service manual you linked contain what appear to be complete schematics for the device. The power supply section is on page 29 and shows the transformer connector, CN1007, with five pins.

Pins 1 and 2 connect to the input of a bridge rectifier formed by diodes D1000 through D1003, which is followed by a 4700μF smoothing capacitor C1243. The negative side of that capacitor connects to circuit ground and the positive side is labelled +9V and feeds into a 78x05 3-terminal linear regulator that generates the +5V Vcc rail.

Pins 3, 4 and 5 connect to a symmetric pair of full-wave rectifiers formed by diodes D1004 through D1007, which generate +16V and -16V that are then fed through a couple more three-terminal linear regulators to derive the +12V Vdd and -12V Vee rails.

If this schematic is correct, your existing backpack transformer is almost certainly just a plain mains-to-low-voltage step-down transformer without any form of inbuilt rectification or regulation; the outputs it feeds to your synth are almost certainly low-voltage AC, not DC. 9VAC is about what I'd expect to see being fed into a bridge rectifier with 9VDC coming out of it; likewise a centre-tapped 30VAC for the +/- 12V rectifiers, with the centre tap connected to pin 4. So although I can't see any actual transformer specs in your service manual, the numbers you've quoted make perfect sense if they're AC voltage outputs. Did you read them off the transformer nameplate?

As well as being twice the voltage of US mains power, most European electricity mains run at 50Hz; the US is all 60Hz. I can see no connections from CN1007 to anywhere but rectifier inputs, so there's no mains-derived timing in this thing. 60Hz is actually easier to smooth than 50Hz, too. Any mains transformer built to work at 50Hz will also work at 60Hz with no issues. All of which means that the different mains frequency shouldn't matter.

So the least invasive solution will certainly be just using a 115 to 230V step-up transformer to drive the backpack transformer you already have. Such a step-up transformer will certainly be easier to source than a replacement for the backpack transformer with floating 9VAC and centre-tapped 30VAC outputs.

Another possibility worth investigating, if you can get inside the backpack transformer's housing, is to check whether its primary winding has a centre tap. If it does, shifting the mains active wire from one end of the primary to the centre tap should let you run the existing transformer off 115V instead of 230V and still get the proper voltages out of it.

But if you're not somebody who is completely comfortable working with mains electricity, just go with the step-up pre-transformer instead. 9V × 1.6A + 2 × 15V × 0.42A comes to 27W, and to keep things comfortable you should use a step-up unit good for at least twice that. Smallest I can find with a quick search is this this 150W unit which should work just fine.
posted by flabdablet at 2:52 PM on May 5, 2020 [7 favorites]

Flabdablet is correct (assuming schematic is accurate.) It's an AC output external power supply. I was going off of the OP's description of the pins "+9" "+16" etc, and didn't download the manual to look at the schematic and double check the info. Please disregard my earlier post.
posted by Larry David Syndrome at 3:13 PM on May 5, 2020 [1 favorite]

Response by poster: @niicholas I did reach out to Waldorf, and I’m still hopeful they have something, I’m just preparing for the likely event they don’t have any spare parts left for 30yo equipment.

@flabdablet & @Larry David Syndrome: Thank you both for the extremely detailed info; that’s exactly what I needed to know, and also I appreciate the advice to keep it simple and use a step up transformer—which is what I will do unless by miracle Waldorf has a replacement part.

Thank you everyone who contributed!!
posted by kyliej at 4:33 PM on May 5, 2020

Best answer: A DIY option would be this transformer, the two 9V coils in parallel to pins 1 and 2, the 15V coils to 3 -4 and 4-5. Built into a project box with a fuse, and a switch if you want, on the mains side and a lead plus connector to your synth, and you're in business.

(Note: for correctly connecting the two 9V coils in parallel you need a multimeter or a small 24V bulb. Connect one of the blue wires with one of the white wires, and connect the bulb between the free white and blue wires. Mark the free white wire. If the bulb lights up/multimeter shows 18V AC mark the OTHER BLUE wire, not the free one going to the bulb/multimeter, if it doesn't light up mark the free blue one. Connect the marked wires together to pin 1, the other two to pin 2)
posted by Stoneshop at 11:21 AM on May 6, 2020 [2 favorites]

Best answer: Oh, good find! I had a bit of a poke around AliExpress as well but gave up sooner than you, obviously.

The 15V windings would need a similar multimeter test (making sure to select the appropriate AC voltage range on the multimeter because trying to measure any AC voltage with a DC voltage meter will show zero).

Pin 4 would get one yellow and one brown joined, but once that joint is made you want 30VAC between the free yellow and free brown wires. If you see 15VAC between pin 4 and either of the free wires, but 0V between the two free wires, swap the other yellow or the other brown (but not both) to the junction on pin 4.

As long as you have 30V between the free yellow and the free brown, it then doesn't matter which of those goes to pin 3 and which to 5.

If you have 0V between the free yellow and the free brown you'd actually still get some power on the +/- 16V rails, but it would have a lot more ripple because the rectifiers would be working in half-wave mode instead of full-wave. That's bad for the filter capacitors and bad for hum, so it's worth getting right even if getting it wrong wouldn't actually short-circuit and cook the transformer like reversing the 9V parallel connection would.

It would be kind of nice to be able to run your synth off the same transformer on your inevitable European stadium tour as you use in the studio back home, just by switching mains live from its orange 115V input wire to its red 230V one.
posted by flabdablet at 12:15 PM on May 6, 2020 [1 favorite]

Response by poster: ohmigosh, y’all are the best! waldorf doesn’t have any more backpacks (us or european), and i did get a basic stepup converter, but i am looking forward to this as a much more elegant and clean solution. thank you both so much!
posted by kyliej at 5:10 PM on May 7, 2020

If you do end up building your own power brick based on Stoneshop's transformer, and you do want to make the input voltage selectable at either 230V or 115V, best practice would be to fit the housing with a fused, switched IEC power entry module and this style of switch for the voltage selector, preferably mounted to the inside of the box with the slider accessible via a small rectangular slot.

You do not want to use a switch that offers opportunities to be knocked into the wrong position with careless handling. The linked kind typically needs a small screwdriver to actuate and should be quite reliable.

These are a double pole double throw switch because they need to be to achieve configurations like flipping dual input windings from parallel to series connection. Stoneshop's transformer doesn't have dual input windings, just a single one with a centre tap, so you can just parallel the two halves of the voltage selector switch for convenient soldering and slightly increased reliability.

I'd also suggest putting the voltage selector switch on the neutral side of the transformer's input rather than the hot side. That way, when it's switched for 115V the unused half of the primary winding will be developing an out-of-phase 115V with respect to neutral rather than an in-phase 230V, which should reduce generated common-mode noise rather than increase it. This doesn't matter a great deal with an R core transformer because that construction pretty much eliminates common-mode capacitive coupling from primary to secondary, but it's nicer all the same.

Wire the power entry module as shown in this instructable, then use white wire to connect the switched neutral on the entry module (the one that the instructable shows as connecting via a white wire to "load") to both the centre pins on the voltage selector switch. Then solder the red wire from the transformer to both pins on the 230V end of the voltage selector switch, the orange wire to both pins on the 115V end, and the black wire to the switched hot on the entry module. The yellow/green screen wire goes to the ground pin on the entry module.

If you use a metal box for this rather than a plastic one, you'll want to wire the box itself to the ground pin on the power entry module as well but I would not connect mains ground to the 0V wire going to the synth. The entire point of this kind of external transformer is to provide a safely isolated low voltage supply that doesn't need a safety ground inside the powered equipment, to avoid creating hum loops. And R core construction makes accidental bridging from mains side to low voltage side very very unlikely, since the windings for each are on completely separate bobbins.
posted by flabdablet at 3:25 AM on May 8, 2020 [1 favorite]

Best answer: I’ve been in touch with Waldorf and a very helpful service technician gave me the following info which jibes with the other advice in the thread. Again thank you all so much; between your advice and Waldorf’s this is now straightforward and easy for me to do. 🙏🏻

Remove transformer and AC cable, insert a 3pin AC connector with ground pin.
Close the gap from the old transformer.
Option 1:
Buy a +15V/-15V/+8Vswitchmode.
Cut the path between 7812 and DC supply. Put +15V at the input of the 7812
Cut the path between 7912 and DC supply. Put -15V at the input of the 7812
Check both 2200uF caps, better exchange them
Cut the path between 7805 and DC supply. Put +8v at the input of the 7805
Check the 4700uf cap, better exchange it
Option 2:
This is a bit better working, but more complex.
Buy a +12/-12/+5V switch mode.
Remove 7812, 7912 and both 2200uF caps. Insert +12V to the output pin 7812 and -12V to the output pin 7912.
Remove the 7805 and 4700uf cap, put +5V at the output point of the 7805, connect ground pin on the mainboard to chassis ground.
Ground the unit with main ground input.
Now the synth should work again.
You have to tune the filter now.
posted by kyliej at 9:34 AM on May 14, 2020

If that was my synth I wouldn't put a switchmode power supply in it, because I would expect doing so to create easily avoidable hum issues. I'd buy Stoneshop's nice R core tranformer from AliExpress and wire that up instead.

The only downside to sticking with the old-school transformer, rectifier, linear regulator stack instead of using a switcher is a bit more heat generation, but since we're only talking 30 watts maximum total supplied, that's not going to be major.

A cheap R core transformer is also inherently safer than a cheap switchmode mains supply because the physical separation between mains wiring and low voltage wiring is inherently better.

I agree with the Waldorf technician's recommendation to replace the high-value filter capacitors because electrolytics do dry out and get tired, but that's all you should need to do inside the synth itself.
posted by flabdablet at 9:39 AM on May 14, 2020 [3 favorites]

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